Apparatus for and Method of Drilling a Subterranean Borehole

ABSTRACT

A control apparatus for use in controlling the fluid pressure in a borehole during drilling of the borehole, the apparatus including an inlet suitable for connection to a pump, a main outlet suitable for connection to a drill pipe, and an overflow outlet suitable for connection to a fluid reservoir, the inlet being connected to the main outlet by a main flow conduit, and the overflow outlet being connected to the main flow conduit by means of an overflow conduit, wherein the overflow conduit is provided with a pressure regulator by means of which the pressure of fluid in the main flow conduit may be maintained at or around a predetermined pressure value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims priority to and thebenefit of co-pending U.S. application Ser. No. 13/262,595, titled“Apparatus for and Method of Drilling a Subterranean Borehole,” whichhas a 371(c) date of Dec. 20, 2011, and which claims priority to PCTapplication PCT/EP2010/054387 filed Mar. 31, 2010, which claims priorityto British patent application GB 0905633.4 filed Apr. 1, 2009, the fulldisclosure of each which is hereby incorporated by reference herein inits entirety.

DESCRIPTION OF INVENTION

The present invention relates to an apparatus for and a method ofdrilling a subterranean borehole, particularly, but not exclusively forthe purpose of extracting hydrocarbons from a subterranean oilreservoir.

The drilling of a borehole or well is typically carried out using asteel pipe known as a drill pipe with a drill bit on the lowermost end.The entire drill pipe may be rotated using an over-ground drillingmotor, or the drill bit may be rotated independently of the drill pipeusing a fluid powered motor or motors mounted in the drill pipe justabove the drill bit. As drilling progresses, a flow of mud is used tocarry the debris created by the drilling process out of the borehole.Mud is pumped through an inlet line down the drill pipe to pass throughthe drill bit, and returns to the surface via the annular space betweenthe outer diameter of the drill pipe and the borehole (generallyreferred to as the annulus). The mud flow also serves to cool the drillbit, and to pressurise the borehole, thus substantially preventinginflow of fluids from formations penetrated by the borehole fromentering into the borehole. Mud is a very broad drilling term, and inthis context it is used to describe any fluid or fluid mixture usedduring drilling and covers a broad spectrum from air, nitrogen, mistedfluids in air or nitrogen, foamed fluids with air or nitrogen, aeratedor nitrified fluids to heavily weighted mixtures of oil or water withsolid particles.

Significant pressure is required to drive the mud along this flow path,and to achieve this, the mud is typically pumped into the drill pipeusing one or more positive displacement pumps which are connected to thedrill pipe via a pipe and manifold known as the standpipe manifold. Whenoperating at a constant speed, the pump displaces a constant volume ofmud per unit time. As a result, if something happens to alter the rateof flow of the mud along the drill pipe, if the pump is operating at aconstant speed, the pressure of the mud in the drill pipe will change.

For example, if a blockage occurs in the drill pipe, the pressure in thedrill pipe will rapidly increase. Alternatively, if the driller drivesthe bit into the formation being drilled with too much weight then thisin itself can cause an increase in the mud pressure in the drill pipe asthe flow out of the drill bit into the annulus is restricted. Anothercommon cause of increasing pressure in the drill pipe is when using adownhole drilling motor. If the torque at the bit exceeds the power ofthe motor, the motor stalls, which results in the standpipe pressureincreasing as fluid flow is reduced through the motor in the stalledcondition. At this stage, the driller has to take care not to just pickup the drill bit, to reduce the torque allowing the motor to restart,but he also has to reduce the pressure in the drill pipe so that themotor does not over speed on restarting.

If the mud pressure in the drill pipe becomes too high, the drillingmotor may fail and/or the drill pipe walls may be damaged. In order toprevent this, the or each pump is provided with a safety valve known asa pop-off valve. If the pressure in the drill-pipe exceeds apredetermined level, the pop-off valve or at least one of the pop-offvalves will be actuated. This stops the drilling process, and relievesthe excess pressure in the drill pipe. Each actuated valve must be resetmanually before drilling can be restarted. It will be appreciated thatthis is highly disruptive to the drilling process, and therefore it isdesirable to avoid fluctuations in drill pipe pressure which are likelyto result in actuation of one or more pop-off valves.

To avoid this, it is known for a drill operator to monitor the mudpressure in the drill pipe, typically by means of a pressure gaugeprovided in the standpipe manifold, and to vary the speed of operationof the pump in order to maintain the mud pressure in the drill pipe atan acceptable level. A disadvantage of operating the drill system inthis way is that it is subject to human error and relies on the operatorreacting promptly to a sudden rise in pressure, and, even if theoperator reacts quickly, because of the inherent inertia of the pump,there is an inevitable time delay before the pump slows to the desiredspeed and before the change in pump speed takes effect downhole.

It will also be appreciated that the bottom hole mud pressure in theborehole is dependant on the flow of mud through the drill pipe. If thisdecreases, because of a plugging of the drill pipe, the application ofexcessive weight to the bit or stalling of the motor, the bottom holemud pressure will decrease. If the mud pressure in the borehole becomestoo low, this may cause the unintended release of hydrocarbon productsfrom the formation.

In an attempt to avoid this, it is also known to control the mudpressure in the borehole by applying a back-pressure to mud exiting fromthe annulus of the borehole. Such a system is, for example, disclosed inUS 2007/0151762. Conventionally, mud from the annulus flows along areturn line through a filter (screens) or series of filters and into amud reservoir. In a first proposal set out in US2007/0151762, a secondpump is provided in a line extending between the return line and the mudreservoir, and the pump controlled to apply the degree of back-pressureto mud in the return line required to maintain the bottom hole pressureat the desired value. If no back-pressure is required, a valve betweenthe return line and the back-pressure pump is closed. This proposal, ofcourse, has a disadvantage that a second pump is required, which adds tothe cost and complexity of the system.

This problem is overcome in the second proposal disclosed inUS2007/0151762 in which the back-pressure is applied using the existingmain rig pump. In this case, a line is provided from the inlet line tothe return line, and a back pressure control valve is provided tocontrol flow of fluid through this additional line. Opening of the backpressure control valve causes a proportion of the mud being pumped bythe main rig pump to be prevented from flowing into the drill pipe anddiverted instead to the return line, where it increases the backpressure in the return line. It is therefore suggested that the bottomhole pressure can be controlled using this valve. Controlling the bottomhole pressure in this way, whilst theoretically possible, would be verydifficult to achieve in practice, at least to any degree of accuracy andwithin a reasonable timescale. Opening the back pressure control valvenot only has the effect of increasing the back-pressure in the returnline, but also decreasing the pressure of mud entering the drill pipe,and this combined effect means that the effect of opening the valve isnot straightforward to predict and, it is likely that numerousiterations and adjustments of the back pressure control valve would berequired to achieve the desired bottom hole pressure.

According to a first aspect of the invention we provide a control systemfor use in controlling the fluid pressure in a borehole during drillingof the borehole, the system including an inlet suitable for connectionto a pump, a main outlet suitable for connection to a drill pipe, and anoverflow outlet suitable for connection to a fluid reservoir, the inletbeing connected to the main outlet by a main flow conduit, and theoverflow outlet being connected to the main flow conduit by means of anoverflow conduit, wherein the overflow conduit is provided with apressure regulator by means of which the pressure of fluid in the mainconduit may be maintained at or around a predetermined pressure value.

By virtue of the provision of such an overflow conduit, when the controlsystem is fitted to drilling system with the inlet connected to the mainrig mud pump, the main outlet connected to the drill pipe, and theoverflow outlet connected to the mud reservoir, the mud pressure in thedrill pipe can be maintained at a generally constant pressure withoutany intervention by the driller. Any of the events describedabove—plugging of the drill pipe, application of excessive weight to thebit or stalling of the motor, the driller does not have to adjust thepump speed to avoid actuation of one or more pop-off valves.

Preferably the pressure regulator comprises a choke which is adjustableto restrict flow of fluid along the overflow conduit to a greater orlesser extent.

Preferably the pressure regulator includes a pressure sensor whichprovides an input signal indicative of the fluid pressure in either themain flow conduit or in the overflow conduit between the choke and themain conduit. Whilst the choke may be manually adjustable, preferablythe pressure regulator also includes an electronic control unit which isconnected to the pressure sensor to receive the input signal from thepressure sensor, the electronic control unit being further connected tothe choke and programmed to generate and transmit to the choke a controlsignal, receipt of which causes the choke to adjust the extent to whichfluid flow along the overflow conduit is restricted. Preferably theelectronic control unit is programmed to monitor the input signal fromthe pressure sensor and if the pressure differs to a specified extentfrom a predetermined value, to calculate and transmit to the choke oneor more appropriate control signals to return the pressure to thepredetermined value. By virtue of the provision of such an electroniccontrol unit, the bottom hole mud pressure may automatically becontrolled and maintained at a desired level.

Further preferably, the electronic control unit is provided with aninput adapted to receive a signal indicating the desired pressure offluid in either the main flow conduit or the overflow conduit betweenthe choke and the main conduit, and is programmed to calculate andtransmit to the choke an appropriate control signal to achieve thedesired pressure. This enables a user to alter the pressure at which thesystem is automatically maintained.

The apparatus may also include a further choke which is located in themain conduit between the overflow conduit and the main outlet and whichis adjustable to vary the degree of restriction of flow of fluid alongthe main conduit.

A valve may be provided in the overflow conduit, the valve being movablefrom an open position in which flow of fluid along the overflow conduitis permitted and a closed position in which the valve acts tosubstantially prevent flow of fluid along the overflow conduit.

The apparatus may include an annulus outlet which is connected to themain conduit between the overflow conduit and the main outlet by anannulus conduit. In this case, preferably yet another adjustable chokeis provided in the annulus conduit. Where a choke is provided in themain conduit, preferably the annulus conduit is connected to the mainconduit between the overflow conduit and the choke in the main conduit.

According to a second aspect of the invention we provide a drillingsystem including drill pipe, a pump, a fluid reservoir and a controlapparatus according to the first aspect of the invention, the inlet ofthe control apparatus being connected to the pump, the main outlet ofthe control apparatus being connected to the drill pipe, and theoverflow outlet of the control apparatus being connected to the fluidreservoir.

Preferably a pressure containment device is provided around the drillpipe, the pressure containment device being capable of containing fluidpressure in the annular region around the drill pipe. In this case, thepressure containment device may be a rotating control device, therotating control device being capable of containing fluid pressure inthe annular region around the drill pipe whilst allowing for rotation ofthe drill pipe about its longitudinal axis.

An annulus return conduit is preferably provided between the annularregion around the drill pipe and the fluid reservoir. In this case,where a pressure containment device is provided, the annulus returnconduit may be provided with a pressure regulator by means of which thepressure in the annular region around the drill pipe may be adjusted.The pressure regulator preferably comprises a choke which is adjustableto restrict flow of fluid along the annulus return conduit to a greateror lesser extent, and may include a pressure sensor which provides asignal indicative of the fluid pressure in the annular region around thedrill pipe. The pressure regulator may also include an electroniccontrol unit which is connected to the pressure sensor to receive aninput signal from the pressure sensor indicative of the fluid pressurein the annular region around the drill pipe, the electronic control unitbeing further connected to the choke and programmed to generate andtransmit to the choke a control signal, receipt of which causes thechoke to adjust the extent to which fluid flow along the annulus returnconduit is restricted. In this case, the electronic control unit ispreferably programmed to monitor the input signal from the pressuresensor and if the pressure in the annular region around the drill pipediffers to a specified extent from a predetermined value, to calculateand transmit to the choke one or more appropriate control signals toreturn the pressure to the predetermined value. Moreover, the electroniccontrol unit may be provided with an input adapted to receive a signalindicating the desired pressure of fluid in the annular region aroundthe drill pipe, and is programmed to calculate and transmit to the chokean appropriate control signal to achieve the desired pressure.

Advantageously, the drilling system further includes an annulus conduitwhich extends from the main conduit to the annular region around thedrill pipe. The annulus conduit is preferably provided with a chokewhich is adjustable to restrict flow of fluid along the annulus conduitto a greater or lesser extent. Further preferably the annulus conduit isprovided with a valve which is operable to substantially prevent flow offluid along the annulus conduit.

According to a third aspect of the invention we provide a method ofoperating a drilling system including a control apparatus, the controlapparatus having an inlet, a main outlet and an overflow outlet, theinlet being connected to the main outlet by a main flow conduit and theoverflow outlet being connected to the main conduit by an overflowconduit, the overflow conduit being provided with a pressure regulator,the drilling system further including a drill pipe connected to the mainoutlet of the control apparatus, a pump connected the inlet of thecontrol apparatus, and a fluid reservoir connected to the overflowoutlet, wherein the method includes the steps of operating the pump topump fluid into the drill pipe via the main flow conduit of the controlapparatus, and operating the pressure regulator to maintain the pressureof fluid in the overflow conduit at or around a predetermined level.

Preferably the pressure regulator comprises an adjustable choke, and themethod includes the steps of adjusting the choke to increase restrictionof fluid flow along the overflow conduit if the pressure of fluid in theoverflow conduit is below the predetermined level or adjusting the choketo decrease restriction of fluid flow along the overflow conduit if thepressure of fluid in the overflow conduit is above the predeterminedlevel.

The control apparatus may also include a further choke which is locatedin the main conduit between the overflow conduit and the main outlet, inwhich case the method may also include the step of operating the mainchoke to alter the extent to which flow of fluid along the main conduitis restricted. The provision of such a choke is advantageous as it meansthat one control needs to be manipulated in order to alter the flow ofmud along the drill pipe, even if two mud pumps are being used.

Preferably the pump is connected to the mud reservoir such thatoperation of the pump causes fluid in the reservoir to be pumped intothe control apparatus.

The drilling system may include an annulus return conduit which extendsbetween the annular region around the drill pipe and the fluidreservoir, the annulus return conduit being provided with a choke whichis adjustable to restrict flow of fluid along the annulus return conduitto a greater or lesser extent, in which case the method may furtherinclude the steps of operating the choke to bring the pressure in theannular region around the drill pipe to or maintain the pressure at adesired level.

The drilling system may further include a main choke which is located inthe main flow conduit between the overflow conduit and the main outletand which is adjustable to vary the degree of restriction of flow offluid along the main conduit, and an annulus conduit which extends fromthe main conduit to the annular region around the drill pipe, theannulus conduit being provided with an annulus choke which is adjustableto restrict flow of fluid along the annulus conduit to a greater orlesser extent, in which case, the method may further include the stepsof, during operation of the pump to pump fluid into the main flowconduit, adjusting the annulus choke to decrease the restriction on flowof fluid along the annulus conduit until the rate of flow of fluid alongthe annulus conduit reaches a predetermined amount, then adjusting themain choke to increase the restriction of flow of fluid along the mainconduit, whilst at the same time adjusting the annulus return choke toincrease the restriction of flow of fluid along the annulus returnconduit. Furthermore, the method may include the step of adjusting themain choke until flow of fluid through the main choke is substantiallyprevented, and then disconnecting the drill pipe from the main outlet.

Alternatively or additionally the method may further include the stepsof, during operation of the pump to pump fluid into the main flowconduit, adjusting the main choke to decrease the restriction of flow offluid along the main conduit, whilst at the same time adjusting theannulus return choke to decrease the restriction of flow of fluid alongthe annulus return conduit, and then. adjusting the annulus choke toincrease the restriction on flow of fluid along the annulus conduituntil the rate of flow of fluid along the annulus conduit issubstantially prevented.

Embodiments of the invention will now be described with reference to thefollowing figures of which,

FIG. 1 shows a schematic illustration of a prior art drilling system,

FIG. 2 shows a schematic illustration of a drilling system including acontrol system according to the first aspect of the invention,

FIG. 3 shows a schematic illustration of a drilling system including asecond embodiment of control system according to the first aspect of theinvention,

FIG. 4 shows a schematic illustration of a drilling system including athird embodiment of control system according to the first aspect of theinvention,

Referring now to FIG. 1, there is shown a schematic illustration of aprior art land-based drilling system including a partially drilledborehole 10 which extends generally vertically into a subterraneanformation, and a drill pipe 12 extending into the borehole 10, leavingan annular space, hereinafter referred to as the annulus 13, between theouter diameter of the drill pipe 12 and the walls of the bore hole 10.The lowermost end of the drill pipe 12 is provided with a bottom holeassembly (BHA) 14 comprising a drill bit and a plurality of sensorspreferably including a pressure transducer which is configured togenerate a pressure signal indicative of the bottom hole fluid pressure.The uppermost section of the borehole is lined with a casing 16, a layerof cement 18 extending between the casing 16 and the sides of the borehole 10.

Whilst the BHA 14 could be provided with a mud motor which is operableto rotate the drill bit, in this example, drilling is achieved byrotating the entire drill pipe 12, using an over-ground drilling motor,or top-drive 30 which is mounted on top of the drill pipe 12. The drillpipe 12 extends out of the bore hole 10 through a blow out preventer(BOP) 34 to the top drive 30.

The system is further provided with a mud reservoir 22, and a mud pump24 which is precharged with mud drawn by a precharge pump from the mudreservoir 22 via a first conduit 25. The mud pump 24 pumps the mud intoa standpipe manifold 28 via a second conduit 26, hereinafter referred toas the main conduit 26. The standpipe manifold 28 is connected to thetop drive 30, and mud pumped into the standpipe manifold 28 passesthrough the top drive 30 and into the drill pipe 12. The standpipemanifold 28 is provided with a pressure sensor, the output of whichconstitutes an indication of the pressure in the drill pipe 12 and isdisplayed at a drillers station.

The mud pump 24 has a safety device called a pop-off valve 62 whichreleases pressure from the main conduit 26 if this is completely pluggedor unintentionally shut in at the standpipe manifold 28. The pop-offvalve 62 is located in a pressure relief conduit 63 which extends fromthe mud pump 24 to the mud reservoir 22. The pop-off valve 62 has to beset manually at the required maximum pressure, which is usuallydependant on the pump specification. If the pop-off valve 62 isactuated, it must be reset manually.

The pump 24 is operated using a variable speed driver which may bemechanical (a diesel engine) or electrical (an electric motor). Thevariable speed driver is controlled by a remote device on the rig floor,with the pump normally being situated elsewhere, typically on theground. During a drilling operation, the driller continuously adjust thedriver, based on the output from the standpipe manifold pressure gauge,in order to maintain the pressure in the drill pipe 12 at the desiredlevel.

Although in this example, the system is described as having a single mudpump 24, it should be appreciated that more than one pump could beprovided. In this case, each pump is provided with a variable speeddriver, and the driller must adjust both drivers in order to maintainthe pressure in the drill pipe 12 at the desired level.

Once pumped down the drill pipe 12, the mud pass through the drill bit14 and into the annulus 13. Having moved up the annulus 13, the mud thenflows into a third conduit 36, hereinafter referred to as the annulusreturn conduit 36, which extends from an uppermost portion of the BOP34. The annulus return conduit 36 extends from the annulus 13 to returnmud back to the mud reservoir 22 via at least one filter 38 and shaker40, by means of which particulate matter such as drill cuttings can beremoved from the returning mud.

The drilling system shown in FIG. 2 is distinguished from such prior artsystems by virtue of the provision of a fourth conduit 42, hereinafterreferred to as the overflow conduit 42, which extends from the mainconduit 26, i.e. from the line between the main mud pump 24 and thestandpipe manifold 28, to the mud reservoir 22. Thus, during operationof the mud pump 24, some of the pumped mud flows into the standpipemanifold 28, and hence the drill pipe 12, whilst some of the pumped mudis returned directly to the mud reservoir 22 via the overflow conduit42.

In the overflow conduit 42 is provided a variable aperture orifice,which is, in this example, a controllable overflow choke 44 which may beoperated to vary the extent to which fluid flow along the overflowconduit 42 is restricted. It will be appreciated that, if the pump 24 ispumping mud at a constant flow rate, closing the overflow choke 44 willdecrease the rate of mud flow along the overflow conduit 42, and hencelead to an increase in the pressure of mud in the overflow conduit 42between the main conduit 26 and the overflow choke 44. As flow of mudbetween the main conduit 26 and the overflow conduit 42, and between themain conduit 26 and the BHA 14 of the drill pipe 12 is, under normalcircumstances, substantially unrestricted, an increase in mud pressurein the overflow conduit 42 results in a corresponding increase in mudpressure in the main conduit 26 and drill pipe 12, and a greaterproportion of the pumped mud will flow into the drill pipe 12.Conversely, it will be appreciated that opening the overflow choke 44will increase the rate of mud flow along the overflow conduit 42, andhence lead to a decrease in the pressure of mud in the overflow conduit42, second conduit 26 and drill pipe 12.

Whilst it would be possible to operate the controllable overflow choke44 manually to bring the mud pressure in the drill pipe 12 to ormaintain it at a desired value, in this example, this is achievedautomatically by means of an electronic control unit (ECU) 48 which isconnected to the overflow choke 44 and which is operable to transmit acontrol signal, which may be pneumatic, hydraulic or electrical, to theoverflow choke 44, receipt of which causes the overflow choke 46 to openor close by a specified amount. Such signal controllable chokes are wellknown in the art.

In this example, a pressure transducer 50 is mounted in the conduit 42between the overflow choke 44 and the main conduit 26, and provides anelectrical output signal which is indicative of the pressure of fluid atthat point in the overflow conduit 42. The pressure transducer 50 isconnected to an input of the ECU 48, and by means of this connection,the output signal from the pressure transducer 50 is transmitted to theECU 48. The ECU 48 is programmed such that if the output signal from thepressure transducer 50 indicates that the mud pressure in the overflowconduit 42 has deviated by more than a predetermined amount from apre-selected value (hereinafter referred to as the set pressure), theECU 48 generates and transmits to the overflow choke 44 an appropriatecontrol signal so that, if the pressure is too high, the overflow choke44 opens so that fluid flow along the overflow conduit 42 is lessrestricted, and if the pressure is too low, the overflow choke 44 closes(partially—not completely) so that fluid flow along the overflow conduit44 is more restricted.

It should be appreciated, however, that the pressure transducer could belocated in the main conduit 26, and therefore provide the ECU 48 with anelectrical output signal which is indicative of the pressure of fluid inthe main conduit 26.

The ECU 48 may be programmed such that the control signal may simplyinclude an instruction to the overflow choke 44 to open or close by apre-determined relatively small degree, and send repeated controlsignals to the overflow choke 44 until the output signal from thepressure transducer 50 indicates that the mud pressure in the overflowconduit 42 is at the desired valve. Alternatively, the ECU 48 may beprogrammed such that the control signal includes not only an indicationas to whether the overflow choke 44 is to open or close, but also by howmuch. In this case, the ECU 48 is programmed to calculate the degree ofopening or closing of the overflow choke 44 required to bring the mudpressure in the overflow conduit 42 to the desired value, and to achievethis by sending an appropriate control signal to the overflow choke 44.

In this example the ECU 48 is programmed to maintain the mud pressure inthe overflow conduit 42 between second conduit 26 and the overflow choke44 around a set pressure of 2000 psi.

By virtue of this arrangement, accurate and reliable control over themud pressure in the drill pipe 12 may be achieved without the need formanual control of the pump speed.

As a proportion of the pumped mud is returned directly to the mudreservoir without passing along the drill pipe 12, it is necessary torun the mud pump 24 at a slightly higher rate (around 10-15% higher)than would be required in prior art systems in which all of the pumpedmud enters the drill pipe 12. If, during drilling, the annulus 13becomes blocked or there is plugging of the drill bit, or if thedrilling motor stalls, there will be a sudden and sharp increase in mudpressure in the drill pipe 12, which will be followed almost immediatelyby a similar increase in mud pressure in the second 26 and fourthconduits 42. This will be detected by the pressure transducer 50, andthe ECU 48 will operate to cause the overflow choke 44 to open and thusrelieve the excess pressure by allowing increased mud flow along theoverflow conduit 42 to the mud reservoir 22. This adjustment can beachieved automatically and very rapidly, and will take effect withoutsignificant delay, in contrast to the prior art method of manualadjustment of the pump speed. As the mud pressure in the drill pipe 12is maintained automatically at a preselected level, actuation of thepop-off valve can therefore be avoided under normal circumstanceswithout the need for the intervention of the driller. The pop-off valvesare therefore provided as pure safety valves for actuation only in theunlikely event of a failure of this pressure control system.

In a preferred embodiment of the invention, the ECU 48 includes afurther input by means of which an operator may alter the set pressureif drilling conditions dictate that a higher or lower pressure of mud inthe drill pipe 12 is required. Preferably this is achieved remotely, forexample from a rig control centre. This input may also be used manuallyto alter the mud pressure in the drill pipe 12, for example, in case offailure of the automatic control system.

In this embodiment of the invention, valves 54, 56 are provided in themain conduit 26 and the overflow conduit 42 respectively. These valves54, 56 are movable between an open position in which flow of fluid alongthe respective conduit is substantially unrestricted, and a closedposition in which flow of fluid along the respective conduit issubstantially prevented. It will be appreciated that by closing thevalve 56 in the overflow conduit 42, the drilling system can be operatedlike a conventional drilling system, with control of the pressure of mudbeing pumped into the drill pipe 12 being achieved by altering the speedof operation of the main mud pump 24.

The provision of the valve 54 in the second conduit 26 means that it isnot necessary to shut down the or each pump 24 during connection of anew length of tubular to the drill pipe 12. The valve 54 can be closedwhilst the new tubular is connected to the drill pipe 12, the mud pumpedby the pump 24 being returned directly to the mud reservoir via theoverflow conduit 42.

Although not essential, this embodiment of drilling system may alsoinclude an electronically controllable main choke 66 in the main conduit26 between the standpipe manifold 28 and the overflow conduit 42. Anelectronic control unit is provided which can be operated from the rigfloor to control operation of the main choke 66, so that the main choke66 can be closed to restrict mud flow into the standpipe manifold 28 oropened to increase mud flow into the standpipe manifold 28. As describedabove, the adjustable overflow choke 44 in the overflow conduit 42 isused to maintain the pressure in the main conduit 26 at a generallyconstant level, and it will be appreciated that the main choke 66therefore provides means for varying the rate of mud flow into the drillpipe 12 without altering the speed of operation of the pump 24. The mainchoke 66 may be operable to close the main conduit 26 completely, i.e.to substantially prevent, rather than simply restrict or impede, flow ofmud along the main conduit 26 or a separate valve may be provided forthis purpose.

One or more flow meters (for example Coriolis flow meters) may beprovided in the drilling system. For example, a flow meter 70 may beprovided in the main conduit 26 between the junction with the overflowconduit 44 and the standpipe manifold 28, or, where provided, the mainchoke 66. The flow meter 70 may be provided either in the conduit 26itself, or in a short looped conduit which extends from a first point toa second point in the main conduit 26. In the latter case, a first valveis preferably provided in the main conduit 26 between the first pointand the second point, and a second valve provided in the looped conduit,such that when the first valve is open and the second valve closed, mudflow is through the main conduit 26 only and there is no flow throughthe flow meter, whereas when the first valve is closed and the secondvalve open, all the mud flowing into the drill pipe 12 flows via theflow meter 70. This flow meter 70 may thus be used to measure the rateof mud flow into the drill pipe 12.

Where both a main choke 66 and flow meter 70 are provided, the drillermay therefore adjust the rate of mud being transmitted to the drill pipe12 by opening or closing the choke 66 with remote control from the rigcontrol floor centre, using the flow meter 70 to ascertain the flowrate.

A flow meter 80 may also be provided in the overflow conduit 42downstream of the choke 44 to measure the rate of mud flow into the mudreservoir 22.

Where both such flow meters are provided the readings from each may becombined to provide an indication of the total output of the mud pump24.

In prior art systems, the volume pumped into the drill pipe 12 may bemeasured by counting the pump strokes of a pump with a stroke counter.With displacement volume being constant it is possible in this way toderive the volume pumped, and this is the usual method of displaying theflow rate at the rig floor control centre. If, however, the pump valvesleak or if the pump loses suction, the derived flow rate may not beentirely accurate. By providing flow meters in the second conduit 26 andthe overflow conduit 42, the flow rates measured using both of theseflow meters can be combined, and the result used to verify the accuracyof the flow rate calculated from the count of pump strokes as in theprior art method.

A second embodiment of the invention is illustrated in FIG. 3, andincludes a rotating control device (RCD) 32. As is conventional in theart, the RCD 32 includes sealing elements which seal against the drillpipe 12 whilst still allowing the drill pipe 12 to rotate, and alaterally extending outlet 32 a located below the sealing elements bymeans of which controlled release of fluid from the annulus 13 via theannulus return conduit 36 may be achieved. Unlike the arrangement shownin FIGS. 1 and 2, the RCD 32 is configured to contain fluid pressure inthe annulus 13.

This embodiment of drilling system also includes an annulus return choke46 which is located in the annulus return conduit 36 between the outlet32 a of the RCD and the filter 38. This choke 46 is also controllable bymeans of an electronic control unit 52 to vary the extent to which mudflow along the annulus return conduit 36 is restricted. If the annulusreturn choke 46 is opened, rapid return of mud from the annulus 13 tothe mud reservoir 22 is permitted, whereas if the annulus return choke46 is closed, flow of mud from the annulus 12 to the mud reservoir 22 isrestricted, and this results in an increase in fluid pressure in theannulus 13, which increase in pressure is contained by the RCD 32.

A flow meter 90 is provided in the annulus return conduit 26 to monitorthe rate of return of fluid from the annulus 13 to the mud reservoir 22.Typically this flow meter is a Coriolis meter and is located between theannulus return choke 46 and the filter 38.

As mentioned above, it will be appreciated that the bottom hole fluidpressure is determined by two factors—namely the rate of flow of mudinto the borehole 10 along the drill pipe 12, and the rate of flow ofmud out of the borehole 10 via the annulus 13. The annulus return choke46 therefore provides a means of controlling the bottom hole mudpressure. In the event that the flow of mud into the annulus is suddenlyreduced during an event such as plugging of the drill pipe 12 orstalling of the motor, to prevent an unwanted sudden drop in the bottomhold pressure, which as mentioned above, could result in the release ofhydrocarbons from the formation, the driller may therefore manuallyoperate the annulus return choke 46 to restrict flow of mud along theannulus return conduit 36 in order to maintain the bottom hole pressureat the desired level.

Also by providing for automatic control of the annulus return choke 46in the same way as for the overflow choke 44, i.e. by providing the ECU52 with an input from a pressure sensor 47 which measures the annulusfluid pressure and programming this ECU 52 to adjust the annulus returnchoke 46 automatically to bring the annulus pressure to a desired value,the system can be set up to provide a constant backpressure on theannulus 13, in addition to a constant pressure in the drill pipe 12.This can assist in maintaining the bottom hole pressure at asufficiently high, if not constant, level to avoid unintended release ofhydrocarbons from the formation during the connection of a new length oftubular to the drill pipe 12.

A further embodiment of drilling system is illustrated in FIG. 4, andthis shows all the features of the system described above andillustrated in FIGS. 1,2, and 3 in combination with some additionalfeatures. In particular, this system includes a fifth conduit 54,hereinafter referred to as the annulus conduit 54, which extends from apoint in the main conduit 26 between the overflow conduit 42 and thestandpipe manifold 28, to the annulus 13 via the bell-nipple 33 and theBOP 34. A valve 60 and a further electronically controllable annuluschoke 63 are provided in the annulus conduit 54, the valve 60 beingup-stream of the annulus choke 63, i.e. between the annulus choke 63 andthe main conduit 26. A further Coriolis flow meter 65 is provided in theannulus conduit 54 between the choke 63 and the BOP 34. Operation of theannulus choke 63 is controlled with an associated ECU 64 in the samemanner as the overflow choke 46, and the annulus choke 63 may be closedto restrict flow of mud from the main conduit 26 to the annulus 13 ormay be opened to increase the diameter of the flow path from the mainconduit 26 to the annulus 13. Ordinarily, during drilling, the valve 60is closed to prevent flow of mud along the annulus conduit 54, and theannulus conduit 54 used only during changeover as described below. Thevalve 60 may, however, be open during drilling,

The valve 60 in the annulus conduit 54, the annulus choke 62 and themain choke 66 may be operated to allow for controlled breaking of theconnection between the top drive 30 and the drill pipe 12, for examplewhen the borehole 10 has become so deep that it is necessary to add anew section of tubing to the top of the drill pipe 12. In this case, thesystem is operated as follows.

With the annulus choke 63 closed, the valve 60 in the annulus conduit 54is opened, and then the annulus choke 63 adjusted, in this example,until the Coriolis flow meter 65 in the annulus conduit 54 indicatesthat mud is flowing along the annulus conduit 54 at a rate of between100 and 150 US gallons per minute. The main choke 66 is then closed on apredetermined closure curve selected to avoid any spikes in bottomholepressure as a result of wellbore storage effects. Whilst this isoccurring, the overflow choke 46 ensures that the pressure in the mainconduit 26 and the annulus conduit 54 is generally constant, which meansthat the rate of flow of mud through the annulus choke 63 into theannulus 13 stays generally constant. Thus, in order to compensate forthe loss in bottomhole pressure due to the closing of the main choke 66,the annulus return choke 46 must also be closed to restrict, but notcompletely prevent, flow of mud along the annulus return conduit 36 andtherefore increase the back pressure on acting on the mud returning fromthe annulus 13. Once the main choke 66 is completely closed, so thatflow of mud into the drill pipe 12 is substantially prevented, the topdrive 30 may be disconnected from the drill pipe 12, and the new sectionof tubing inserted into the drill pipe 12. Whilst this is occurring, thepump speed may be reduced to reduce the amount of mud being pumpedstraight back into the mud reservoir 22 via the overflow conduit 42,whilst maintaining the desired flow rate along the annulus conduit 54.

By virtue of this simultaneous control of the main choke 66 and theannulus return choke 46, the bottom hole pressure may be kept at agenerally constant level during the controlled breaking of theconnection between the top drive 30 and the drill pipe 12. Such controlof the bottom hole pressure is achievable because of the constantpressure supply to the main choke 66 facilitated by the overflow conduit42 and control of the overflow choke 44. This ensures that the effect ofclosing the choke 66 is generally predictable, and based on thispredictable response, it is possible to drive the annulus return choke46 to maintain a constant bottom hole pressure during the connectionprocess.

After reconnection of the top drive 30, in order to restart drilling andhence pumping of mud into the drill pipe 12, the process is reversed,with the main choke 66 and the annulus return choke 46 being openedtogether. It should be appreciated, however, that the main choke 66 isopened at a different rate to that at which it was closed, as the fluiddynamics of recommencing mud flow into the drill pipe 12 have adifferent effect on the bottomhole pressure than reduction in mud flowinto the drill pipe 12 during the disconnection process.

When used in this specification and claims, the terms “comprises” and“comprising” and variations thereof mean that the specified features,steps or integers are included. The terms are not to be interpreted toexclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately, or in any combination of such features, be utilised forrealising the invention in diverse forms thereof.

1. A method of drilling a borehole using a drilling system including acontrol apparatus, the control apparatus having an inlet, a main outletand an overflow outlet, the inlet being connected to the main outlet bya main flow conduit and the overflow outlet being connected to the mainconduit by an overflow conduit, the overflow conduit being provided witha pressure regulator, the drilling system further including a drill pipeconnected to the main outlet of the control apparatus, a pump connectedthe inlet of the control apparatus, and a fluid reservoir connected tothe overflow outlet, wherein the method includes the steps of operatingthe pump to pump fluid into the drill pipe via the main flow conduit ofthe control apparatus, characterized that the method further includesoperating the pressure regulator to maintain the pressure of fluid inthe main flow conduit at or around a predetermined level while drillingthe borehole.
 2. A method of drilling a borehole according to claim 1wherein the pressure regulator comprises an adjustable overflow choke.3. A method of drilling a borehole according to claim 1 wherein thecontrol apparatus also includes a further choke which is located in themain conduit between the overflow conduit and the main outlet, and themethod also includes the step of operating the main choke to alter theextent to which flow of fluid along the main conduit is restricted.
 4. Amethod of drilling a borehole according to claim 1 wherein the pump isconnected to the mud reservoir such that operation of the pump causesfluid in the reservoir to be pumped into the control apparatus.
 5. Amethod of drilling a borehole according to claim 1 wherein the drillingsystem includes an annulus return conduit which extends between theannular region around the drill pipe and the fluid reservoir, and theannulus return conduit is provided with a choke which is adjustable torestrict flow of fluid along the annulus return conduit to a greater orlesser extent, and the method further includes the steps of operatingthe choke to bring the pressure in the annular region around the drillpipe to or maintain the pressure at a desired level.
 6. A method ofdrilling a borehole according to claim 5 wherein the drilling systemfurther includes a main choke which is located in the main flow conduitbetween the overflow conduit and the main outlet and which is adjustableto vary the degree of restriction of flow of fluid along the mainconduit, and an annulus conduit which extends from the main conduit tothe annular region around the drill pipe, the annulus conduit beingprovided with an annulus choke which is adjustable to restrict flow offluid along the annulus conduit to a greater or lesser extent, themethod further including the steps of, during operation of the pump topump fluid into the main flow conduit, adjusting the annulus choke todecrease the restriction on flow of fluid along the annulus conduituntil the rate of flow of fluid along the annulus conduit reaches apredetermined amount, then adjusting the main choke to increase therestriction of flow of fluid along the main conduit, whilst at the sametime adjusting the annulus return choke to increase the restriction offlow of fluid along the annulus return conduit.
 7. A method of drillinga borehole according to claim 6 wherein the method further includes thestep of adjusting the main choke until flow of fluid through the mainchoke is substantially prevented, and then disconnecting the drill pipefrom the main outlet.
 8. A method of drilling a borehole according toclaim 5 wherein the drilling system further includes a main choke whichis located in the main flow conduit between the overflow conduit and themain outlet and which is adjustable to vary the degree of restriction offlow of fluid along the main conduit, and an annulus conduit whichextends from the main conduit to the annular region around the drillpipe, the annulus conduit being provided with an annulus choke which isadjustable to restrict flow of fluid along the annulus conduit to agreater or lesser extent, the method further including the steps of,during operation of the pump to pump fluid into the main flow conduit,adjusting the main choke to decrease the restriction of flow of fluidalong the main conduit, whilst at the same time adjusting the annulusreturn choke to decrease the restriction of flow of fluid along theannulus return conduit, and then adjusting the annulus choke to increasethe restriction on flow of fluid along the annulus conduit until therate of flow of fluid along the annulus conduit is substantiallyprevented.
 9. A method of drilling a borehole according to claim 1wherein the method includes the steps of adjusting the choke to increaserestriction of fluid flow along the overflow conduit if the pressure offluid in the overflow conduit is below the predetermined level oradjusting the choke to decrease restriction of fluid flow along theoverflow conduit if the pressure of fluid in the main flow conduit isabove the predetermined level.